The present invention generally relates to laser drilling and particularly relates to microfilter design for laser drilling systems producing multiple sub-beams for parallel drilling operations.
Material ablation by pulsed light sources has been studied since the invention of the laser. Etching of polymers by ultraviolet (UV) excimer laser radiation in the early 1980s led to further investigations and developments in micromachining approaches using lasersxe2x80x94spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers. A recent article entitled xe2x80x9cPrecise drilling with short pulsed lasersxe2x80x9d (X. Chen and F. Tomoo, High Power Lasers in Manufacturing, Proceedings of the SPIE Vol. 3888, 2000) outlines a number of key considerations in micromachining. Other recent patents of interest include the following:
U.S. Pat. No. 6,252,714, xe2x80x9cDiffractive homogenizer with compensation for spatial coherence,xe2x80x9d describes a diffractive homogenizer for receiving a beam of laser energy and producing a desired illumination pattern in a target plane. The homogenizer is made up of a plurality of diffractive sub-elements, each of which contributes to all or a portion of the desired image. By combining the contributions of many sub-elements to form the final image, a homogenizing effect is realized. In preferred embodiments, the sub-elements are designed to compensate for the finite spatial coherence of the incident laser beam and to control the numerical aperture distribution of the transmitted light. Each sub-element is composed of a large number of discrete pixels, each of which alters the phase of radiation passing therethrough by a selected amount. The pixel arrangement is chosen; using computer modeling and optimization techniques, such that the interference pattern created by the collective pixels in a sub-element makes up the desired image (or a portion thereof). A technique is also provided for reducing the intensity of the image formed by a selected sub-element, which may be located in a laser xe2x80x9chot spotxe2x80x9d, by randomizing a selected percentage of the pixels located in that sub-element. This diffractive homogenizer is useful in various laser ablation and annealing, and other laser material processing applications.
U.S. Pat. No. 6,243,209, xe2x80x9cMethod and apparatus for providing rectangular shape array of light beams,xe2x80x9d describes a linear array of equal intensity optical beams transformed into a rectangular array of equal intensity optical beams, while the intensity of each beam is kept nearly constant. The transformation is performed using an optical element that has two coatings on the front surface and a reflective coating on the opposing back surface. The front surface is partially coated with a reflective coating and partially coated with an anti-reflective coating. The beams are incident upon the front surface, with some of the beams incident on each of the two different coatings on the front surface. The beams incident on the front surface are specularly reflected. The remaining beams are transmitted through the optical element to the back surface, reflected from the back surface, and transmitted back up through the optical element and exit from the front surface. The exiting beams are thus shifted laterally and transversely to define the desired rectangular array. The index of refraction, thickness of the optical element, and the incident angle of the beam are selected to achieve the desired arrangement of beams.
U.S. Pat. No. 6,236,509, xe2x80x9cDiffractive optical system with synthetic opening and laser cutting device incorporating this system,xe2x80x9d describes an optical device for focusing a light beam. The device includes a Fourier diffractive element that can separate an incident beam into n beams along n directions that are symmetric about an optical axis. The device also includes a diffractive element including a Fresnel lenses capable of refocusing the n beams onto the optical axis. The device may be used with lasers and laser cutting devices.
U.S. Pat. No. 6,025,938, xe2x80x9cBeam homogenizer,xe2x80x9d describes a beam homogenizer that minimizes undesired intensity variations at the output plane caused by sharp breaks between facets in previous embodiments. The homogenizer includes a hologram made up of irregularly patterned diffractive fringes. An input beam illuminates at least part of the hologram. The hologram transmits a portion of the input beam onto an output plane. In doing so, the energy of the input beam is spatially redistributed at the output plane into a homogenized output beam having a pre-selected spatial energy distribution at the output plane. Thus, the illuminated portion of the output plane has a shape predetermined by the designer of the homogenizer.
U.S. Pat. No. 5,566,024, xe2x80x9cBeam separation control and beam splitting by single blazed binary diffraction optical element,xe2x80x9d describes two sets of two single blazed binary diffractive optical elements that form a beam separation control apparatus for expanding two closely spaced parallel beams into two wider spaced parallel beams or for contracting two wider spaced parallel beams into two closely spaced parallel beams. Four sets of two single blazed binary diffractive optical elements form a beam separation control apparatus for separating two closely spaced parallel beams into two wider spaced parallel beams for possible modulation or other optical effect, then returning the two beams to be closely spaced and parallel. A set of two adjacent and opposite single blazed binary diffractive optical elements can form a beam splitting apparatus or a beam combining apparatus.
Ultrafast lasers generate intense laser pulses with durations from roughly 10xe2x88x9211 seconds (10 picoseconds) to 10xe2x88x9214 seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10xe2x88x9210 seconds (100 picoseconds) to 10xe2x88x9211 seconds (10 picoseconds). Along with a wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications, short pulse lasers are also useful in milling or drilling holes in a wide range of materials. In this regard, hole sizes in the sub-micron range are readily drilled by these lasers. High aspect ratio holes are also drilled in hard materials; applications in this regard include cooling channels in turbine blades, nozzles in ink-jet printers, and via holes in printed circuit boards.
Parallel processing of laser-milled holes is a key technique for increasing throughput in laser micromachining. Beamsplitting devices (beamsplitters) such as diffractive optical elements (DOEs) are used in laser micromachining to divide a single beam into multiple beams and thereby achieve parallel machining. However, such use of beamsplitters introduces technical challenges in hole geometry requirements and in the ability to produce consistent results. Such challenges need to be overcome in order to maintain consistency and repeatability in laser milling.
lnkjet nozzle design, construction, and operation are all important factors in providing high quality inkjet print resolution. Inkjet nozzle designs, which typically include specific patterns of many ink jet holes, which in turn are also specific defined geometries, provide the templates for nozzle holes drilled in a thin foil or polymer to a particular shape. Each nozzle hole includes an input section, a shaped section and an exit hole section, and each exit hole section is preferably cut with a high degree of precision respective to the design pattern. In a particular nozzle inconsistency in nozzle hole shape leads to inconsistent expulsion of inks among the individual holes in an inkjet nozzle, which negatively affects print resolution. Therefore, imperfections in the shape of the inkjet nozzle holes respective to the design pattern negatively impact print quality.
When a DOE is used to produce multiple sub-beams for parallel machining, generally there is variation in beam strengths among the sub-beams, i.e., some sub-beams are more intense than the average sub-beam strength and some are weaker than the average sub-beam strength. The variation is caused by the design and/or fabrication imperfections of the DOE. The beam strength variation among the sub-beams leads to size variations among the machined geometries. Stronger sub-beams tend to machine larger sizes. If the beam strength variation is too large that the machined geometries exceed the product specification, and thus means must be found to reduce the beam strength variation among the sub-beams of the DOE.
Microfilters are used in equalizing sub-beam intensities to enable a parallel process laser drilling system to drill consistent workpiece geometries. One important application for such a use is in inkjet nozzle hole manufacture. However, the respective microfilter is also subject to factors derived from manufacturing errors and design limitations. In this regard, microfilters do not, as delivered, predictably sufficiently equalize the intensities of sub-beams in parallel process laser drilling systems because the microfilters are designed with inaccurate sub-beam intensity data. This data is inaccurate insofar as it is theoretical as based on design inputs of the beamsplitter, rather than being based on empirical measurements of actual sub-beam intensities. Current technology does not provide a way to empirically measure the intensities of subs beams to a level of accuracy acceptable for use in designing a microfilter for use in precision parallel laser drilling.
What is needed is a way to improve accuracy of measuring relative beam intensities in parallel process laser drilling system so that the design input parameters for microfilter design will be more accurate and so that microfilter designs and the resultant microfilters will improve to provide sufficiently balanced and homogeneous parallel subbeams for consistent hole manufacture. The present invention provides a solution to this need.
According to the present invention, a microfilter design system for use with a laser drilling system producing multiple sub-beams for parallel. drilling operations includes an optical intensity detector illuminated by the multiple sub-beams of the laser drilling system. An analysis module operates the optical intensity detector to produce intensity measurement data for each of the multiple sub-beams. A memory operable with a data processing system stores the intensity measurement data for analysis.
The present invention provides a method for providing sub-beam impingement intensity control from a set of sub-beams generated from a parallel process laser system and impinged upon a target, where at least two of the sub-beams having an impingement separation at the target of less than about 260 microns by measuring the impingement intensity of each sub-beam to generate a sub-beam intensity measurement and attenuating the intensity of each sub-beam in response to the measurement.
In preferred form, the invention uses, in the measuring step, a scanning diode and blocking plate with an aperture positioned to pass the sub-beam whose intensity is being measured to a target point, while at the same time blocking all adjacent sub-beams from passing to impinge upon the target point being measured.
As should be readily appreciated, the invention also provides a laser cutting apparatus, such as used in manufacturing an inkjet nozzle, which uses a microfilter derived from the above steps.
A number of advantages are provided with the invention. By providing a way to improve the accuracy of measuring relative beam intensities in a parallel process laser drilling system, further derived benefits of improved microfilter design parameters and improved mirofilter design are readily realized. A solution approach is also achieved for compensating for fabrication errors and minor defects in diffractive optical elements. An approach is also derived for compensating for final intensity variations between sub-beams emitted from a diffractive optical element. Print resolution in inkjet printers is also realized when the inkjet nozzles of the printer are manufactured with the benefit of the invention.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.